Electrochromic materials are now known which change their optical properties in response to the application of an electric current or potential. A variety of solid-state inorganic electrochromic layers have thus been devised including those effecting color change based on the dual injection of electrons and ions, particularly Group VI-B oxides, such as WO.sub.3 and MoO.sub.3. In general, these electrochromic devices will include a structure which consists of sequential layers including a layer of an electrically conductive material, an electrode formed from a layer of electrochromic material, an ion-conducting layer, a counterelectrode layer, and another electrically conductive layer. In a first condition of these electrochromic devices, each of the aforementioned layers is optically transparent such that a majority of the optical energy incident on the device will be transmitted therethrough. Upon the application of an electrical potential across these layers, however, the optical properties of the electrochromic material will change such that the electrochromic layer will become less transparent, thereby preventing the transmission of much of the optical energy therethrough.
These electrochromic devices have a significant number of potential uses, particularly in controlling the transmission of optical energy through windows, particularly the large windows of office buildings and other such structures. The efforts to capitalize on these benefits have been developing for many years, and improved electrochromic devices are now being devised.
Among the most significant parameters in developing such electrochromic devices, however, have been to improve the transmissive properties of each of the layers, including the counterelectrode layer itself. In general, the most widespread material used for these counterelectrode layers has been vanadium oxide (V.sub.2 O.sub.5). Spindler, U.S. Pat. No. 5,209,980, discloses transparent counterelectrodes which have been developed. In this case, alternative transparent complementary counterelectrodes are disclosed using films such as indium hexacyanoferrate, gadolinium hexacyanoferrate, and gallium hexacyanoferrate. These films are produced by electroplating onto a conductive surface, such as a tin-oxide-coated glass substrate. In each of these electrochromic devices, upon the application of an electrical potential across the two electrodes, ions which are present in the electrolyte are absorbed by one of the electrodes producing a change in color or transmissivity of the electrode. Reversal of the current in the circuit reverses the chemical reaction, and the changed electrode then reverts to its original condition. The purpose of the counterelectrode is to "store" a large quantity of these ions, such as lithium ions or protons, and associated electrons in a transparent state. That is, the bleached state transmission of most of these electrochromic devices tends to be limited by the transmissivity of the counterelectrode itself. Therefore, the search has continued for counterelectrodes which are more highly transmissive when "fully charged" than has previously been the case. A slight improvement in these transmissive properties of the counterelectrode can have a dramatic effect upon the bleached state transmission of the electrochromic device itself.
Vanadium oxides and lithium vanadium oxides are known to exist in different forms. Thus, vanadium oxide can exist in the alpha (.alpha.) crystalline form or amorphous state. In the prior art devices employing lithium counter ions, it has been shown that the room-temperature addition of lithium to .alpha.-V.sub.2 O.sub.5 having a molar ratio of lithium to vanadium of about 1 yields epsilon (.epsilon.) form of the compound, or .epsilon.-Li.sub.x V.sub.2 O.sub.5. In addition, it is also known that this epsilon form of Li.sub.x V.sub.2 O.sub.5 converts to a gamma (.gamma.) form of the compound, or .gamma.-Li.sub.x V.sub.2 O.sub.5 at a temperature of about 300.degree. C. A stability diagram showing these forms of vanadium oxide is set forth in FIG. 1. (See Murphy, D. W. et al., "Lithium Incorporation by Vanadium Pentoxides," Inorganic Chemistry, Vol. 18, pp. 2000-2803 (1979)). The search has, continued for counterelectrodes of this type having better transmissive properties.